1. Field Of The Invention
The invention relates to a device for removing soil particles from the plate cylinder of an offset printing machine, which device extends over the entire length of the plate cylinder and engages the plate cylinder in the region between the ink transfer to the rubber cylinder and the dampening.
2. Description Of The Prior Art
In offset printing, the plate cylinder becomes repeatedly soiled. The soils may be comprised of fibers of paper or cardboard from the material being printed on, balls or clumps of printing ink, or dust from the air. They can give rise to unprinted spots on the surface being printed. To eliminate this problem, either the printing machine must be stopped for hand removal of the soil particles or else a device must be applied against the rotating plate cylinder which device removes the soil particles. Stopping the press reduces the output and increases rejects which necessarily result whenever the press is stopped. On the other hand, engaging the running press with cleaning tools is hazardous to the operator, and is indeed prohibited. Further, because this (running-press) technique can only be carried out from the ink-application side, it results in an undesirable partial removal of ink.
Accordingly, there has been much research directed at means of removing soils from the plate cylinders of offset printing machines which means are free from these disadvantages. In this connection, solutions have become known of the types described, e.g., in Ger. OSs 25 22 743, 27 15 445, 28 50 448, 34 10 376, and also 36 20 156. The claimed matter of the first four cited German references involves devices in which a guide means is provided which is parallel to the rotational axis of the plate cylinder in the ink-free region of the plate cylinder, and which extends over the entire length of the plate cylinder. The guide means bears a driven cleaning implement. These known devices have not found acceptance in the art, however. The reasons for this are, first, that the space requirements for the guide means, cleaning implement, and drive are so great that the device is virtually impossible to accommodate in the very narrow region of the dampening system, and secondly, that the cleaning implement must be adjusted with each application, which requires high accuracy because of the sometimes only local point contact between the implement and the cylinder; and further, when soil particles are present at several locations they must be removed sequentially, which necessitates an extending cleaning time and therefore a high reject rate (amount of printed work which must be discarded). Ger. OS 3,620,156 discloses a device wherein a cleaning implement extends over the entire length of the plate cylinder, the implement having the form of an application cylinder of the dampening system. However, this device also does not produce the desired result. The means whereby the application cylinder is enabled to function as a cleaning implement are mechanically complex. Furthermore, the cleaning action is unreliable, because if the plate cylinder and application cylinder are not exactly parallel the soil particles will not be removed over the entire length of the cylinder. In addition, the force of the application cylinder against the plate cylinder is relatively low, whereby the result may be that the soil particles are only mashed down instead of being removed.